Coolant plate structures for fuel cell stack assemblies typically include a metal serpentine coolant flow tube which is embedded in a conductive plate formed from a carbon/binder mixture. The conductive plates have been formed from carbon particles which are essentially spheroidal in configuration; and the binder is a fluorocarbon resin which is hydrophobic, and which imparts hydrophobic characteristics to the coolant plate. The plates are homogeneous mixtures of carbon and binder in the through plane.
Problems have arisen in connection with the prior art coolant plates, which problems relate generally to the stability of the plates and their ability to transfer heat to the coolant tubes under stack operating conditions. The use of spheroidal carbon particles as the conductive component of the coolant plates has resulted in the need for relatively high percentages of hydrophobic resin binder in order to form a cohesive component which will endure under stack operating conditions. The high binder content results in a coefficient of thermal expansion (CTE) for the composite plate which is sufficiently different from the CTE of other stack components that thermal stress will occur when the stack is operated.
Another problem that resides in the prior art coolant plate assemblies relates to the ability of the carbon particle-binder mixture to adhere to the serpentine metal coolant tube. In order to provide for good heat transfer from the plate to the coolant, a durable intimate bond is desired between the carbon particles and the metal tube. Such a bond has not, however, been consistently produced by the carbon particle-hydrophobic binder system used in the prior art. One solution to the bond problem has been to coat the metal coolant tube with a resin layer so that the resin component in the plate will bond to a similar resin on the tube. This approach results in an acceptable bond but produces less efficient heat transfer to the coolant because of the resin layer on the coolant tube.
Still another problem attendant to the prior art coolant plate assemblies relates to the difference in the CTE of the metal tube as compared to the carbon particle component of the assembly. The dilemma facing the stack designer involves the need to minimize thermal stress when a graphite-binder/metal composite component contacts a graphite-binder component. The CTE of the metal component is different from the CTE of the graphite-binder components so the designer is faced with the problem of how to tailor the graphite-binder portion of the coolant plate to so as to minimize thermal stress between the various components of the stack.